1. Field of the Invention
The present invention relates to an electronic driving circuit for driving passive elements such as heating resistors, a thermal head provided with heating resistors, a manufacturing method of the thermal head, and heat sensitive recording apparatus using the thermal head.
2. Description of the Prior Art
FIG. 1 is a circuit diagram showing a conventional thermal head of a thick film alternately reading type system disclosed in, for example, a Japanese Patent Publication (Kokai) No. 51-81137. In the figure, 101 is a recording head, 102 is a line memory for recording information signal (represented by A) quantized in time for one scanning line, 103 is a switch for switching a driving signal (B or C) for the line memory 102, 104 is a memory represented by a shift register of a serial input and parallel output type which stores one half of the one scanning line recording information signals and outputs these signals in parallel, and which is connected through not-shown switching elements (represented by transistors) to one-side leads of the recording head 101. Also, 105 is a switch for selecting an input signal to the memory 104, 106 is a switch for switching a driving clock signal (D or F) for the memory 104, 107 is a memory for recording another half of the recording information signals for one scanning line, 108 is a switch for switching a driving clock signal (E or F) for the memory 107, 109 is a switch for selecting a common terminal from common terminals which are connected to an odd group and an even group of the other-side leads, in the sequence of the arrangement of the recording head a and 110 denotes inverse current preventing elements represented by a semiconductor diode array.
Such a thermal head is widely used in the field of facsimile machines, printers, plotters and so forth, because of the simple printing system.
Apart from the above, as conventional thick film thermal heads, there are those disclosed in Japanese Patent Publication (Kokai) No. 51-58958, Japanese Patent Publication (Kokai) No. 51-81138, Japanese Patent Publication (Kokai) No. 51-81138, Japanese Patent Publication (Kokai) No. 51-115838, and Japanese Patent Publication (Kokai) No. 51-115839.
In the thermal head of the thick film alternately reading type shown in FIG. 1, heating resistors (not shown) of the reading head 101 are driven by a circuit formed by the inverse current preventing circuit 110 formed by the diode array a the line memory 102, the memory 104, the memory 107 driven the switching elements which are driven by the recording information. Therefore, when at least one line is to be printed, it is necessary to drive the heating resistors by each half of the recording information signals, namely, by switching the line memory 102 and the memory 107 (which are buffer memories). The data transfers for printing one line of the recording information are two. This is because the diode array 110 is separated into two groups, so that a switching by the switch 109 is necessary. Here, the current to be conducted through one diode is relatively large in comparison with the current through one transistor, so that, when plural diodes are driven at the same time, the switching requires a large current. Accordingly, if the switching speed is increased, the diode array 110 or the switch 109 (formed by, for example transistors) will be broken due to the generation of spike noises caused by the large current switching. Therefore, it is impossible to effect the high speed switching. In conclusion, a current to be passed through only one diode can be switched at a high speed, however, the elements which can realize the switch 109 must always be limited to be able to switch a large current at a high speed.
Further, the printing data for one line must be separated and stored into the buffer memories 102 and 107 and must be combined again to form the driving information. To this end, there is known a thermal head disclosed in the above-mentioned Japanese Patent Publication (Kokai) No. 59-123364 and the Japanese Patent Publication (Kokai) No. 59-123365.
FIG. 2 is a circuit diagram showing a conventional thermal head disclosed in, for example, Japanese Patent Publication (Kokai) No. 59-123365. In the figure, 111 is a heating element group consisting of n heating elements 111a to 111n which are arranged adjacent to each other, 112 is a transistor array consisting of n/2 transistors 112a which are heating element driving buffer elements, each of the transistors 112a being connected to one pair of the adjacent two heating elements in the heating elements 111a to 111n in the heating element group, and 113a and 113b are first and second common electrodes to which voltages V.sub.1 and V.sub.2 are applied at different times, the first heating element 111a of the above-mentioned heating element group 111 being connected to the first common electrode 113a, the adjacent two of the second to (n-1)-th heating elements 111b to 111n-1 being sequentially connected to the second or the first electrode 113a or 113b, and the n-th heating element 111n being solely connected to the second or the first common electrode 113a or 113b. 114 is an inverse current preventing diode, 115 is an n-bit shift register for holding the printing data to the heating element group 111, 115a and 115b are clock input terminals and data input to the shift register 115, 116 is a multiplexer for selecting one output from two outputs in the parallel outputs of the shift register 115 to switch a transistor 112a, 116a is a terminal for receiving a selecting signal for selecting one or the other group of adjacent two AND gates in the multiplexer 116, and 116b is a terminal for receiving a strobe signal for determining the driving period of the heating resistors 111. To the terminal 116b, a driving signal "H" is applied in the driving period, and a signal "L" is applied when all of the heating resistors are not to be driven. 116c and 116d are inverters.
As shown in FIG. 2, the n-bit printing data for one line are stored in the shift register 115, and the heating element driving data can be easily obtained by the multiplexer 116 so that the data division and reconstruction, which were necessary in the prior art shown in FIG. 1, are not necessary in the prior art of FIG. 2.
In FIG. 2, however, there are problems as follows.
First, since the diodes in the diode array 114 are driven by switching the signals applied to the terminals C1 and C2 by using transistor switches (not shown), and since a current to be conducted through each diode is relatively large, a large current is passed through the switching transistors when the switching is carried out. Therefore, a high speed switching can not be effected.
Second, the transistor array 112, the shift register 115, and the multiplexer 116 are formed as an integrated circuit in an IC chip. Accordingly, to realize the thermal head shown in FIG. 2, as shown in FIG. 3A which is a plan view of the thermal head including the IC chip, the heating resistors 111 must be arranged on the center of a substrate 311, the diode array 114 must be arranged on the one side of the substrate 311, and the IC chip 312 including the shift register 115, the multiplexer 116, and the transistor array 112 must be arranged at the other side of the substrate 311 opposite to the diode array 114 with respect to the heating resistors 111.
As a result, either the diode array side or the IC chip side becomes the output portion for the recording paper 313 which is in contact with the heating resistors so that the image cannot be seen immediately after printing FIG. 3B shows a side view of the thermal head shown in FIG. 3A. In FIG. 3B, 111 is the heating resistor, 311 is the substrate of the thermal head, 313 is a recording paper, and 314 is a platen roller for carrying the recording paper 313. As can be seen from FIG. 3B, users can see the printed paper only after the portion of the recording paper 313 printed by the heating resistors 111 passes the portion of the IC chip 312 or the portion of the diode array 114, depending on the direction of the paper through the thermal head printer. Accordingly, there are disadvantages in that it takes a long time to adjust the position of the recording paper 313 and, if the positioning of the recording paper is wrong, a considerable amount of the recording paper 313 must be discarded before repositioning it.
On the other hand, with respect to the shape of a printing dot of these known thick film alternately reading type thermal heads shown in FIG. 1 and FIG. 2, the shape of the printing dots for one horizontal line is formed by a pair of two dots as shown in FIG. 9A described later, however, since the switching speed of the switch 109 in FIG. 1 or the switching speed of the signals applied to the terminals C1 and C2 can not be high, the dot width in the sub scan direction is so long that the level of the quality of the picture does not allow for printing of graphs and so forth. To prevent this, it may be possible to drive the switch 109 in FIG. 1 or to switch the signals applied to the terminals C1 and C2 in FIG. 2 and to transfer data many times during one line printing so that, as shown in FIG. 9B, it may be anticipated that the width of the printed dots may become within the allowed level. However, to realize this, since the switch 109 or the signal applied to the terminals C1 and C2 is switched with a large current, it is difficult to realize the dot shape shown in FIG. 9B when the printing speed is high. Also, since the printing data transfer is effected so frequently, it is difficult to realize the dot shape shown in FIG. 9B. Accordingly, by the prior art thermal head shown in FIG. 1 or FIG. 2, the shape of the printing dots is that shown in FIG. 9A when the printing period is around 1.25 ms.
As other conventional examples, Japanese Patent Publication (Kokai) No. 5-8428 and Japanese Patent Publication (Kokai) NO. 5-8429 are known, however, these are the same as above in the points that each of these also uses a diode array so that the switching is effected with a large current.
In conclusion, since the conventional thermal heads are constructed as above, there were problems in that the high speed switching element driving with a large current is difficult, the quality of the picture is not good, and the picture immediately after printing can not be seen.
The above-described problems in the prior art reside not only thermal heads but also other electronic parts having the similar construction as the thermal head.